Self-compensated plate current oscillator



Nov. 1l, 1952 w. s. ERWIN 2,617,856

SELF-COMPENSATED PLATE CURRENT oscILLAToR Filed oct.. 1e,- 194e PatentedNov. 11,1952

SELF-COMPENSATED PLATE CURRENT.`

QSCILLATOR Wesley S. Erwin, Detroit, Mich., assigner to General MotorsCorporation, Detroit, Mich., a cor` poration of Delaware ApplicationOctober 16, 1946, Serial No. 703,678

l The present invention relates to electronic oscillators and moreparticularly to oscillators that are tunable over a range of frequenciesand are used to drive an electrical or electro-mechanical load.

More specifically this invention relates to a means for compensatingno-load plate current variations as an oscillator is tuned over afrequency range.

It is well known in the radio art that a parallel tuned circuit atresonant frequency has a resistive impedance in ohms equal to L/RC. ,Inthis equation, L is the inductance of the circuit in henries, C is thecapacitance of the condenser in the circuit in farads, and R is theohmic resistance of the circuit in ohms. Thus,l as the Value of either Cor L is varied to tune the resonant plate circuit of an oscillatorover arange of frequencies, the resistive impedance of the plate circuit isvaried with a resulting change in the plate current. If the Value of Ris changed as a result of resistance reected from the load, this willalso result in a change of plate current. This latter change in platecurrent is very useful in determining the impedance of a load coupled tothe oscillator.

In order to determine the amount of change in plate current attributableto the loading of the plate circuit, some means of adjustment must beavailable to compensate forV that change in plate current caused by thetuning of the circuit. In the prior art, it has been the practice torecord the no-load plate current at the frequency under considerationand subtract this plate current from the actual plate current of theloaded oscillator in order to determine the change in current resultingfrom the load. In order to use this method of compensation it isnecessary to utilize a plate current meter having a considerable currentrange with resulting low sensitivity. If the load to be measured issmall so as to cause only a minute change in plate current; `it isapparent that the inaccuracies of the prior art measuring methods aresuch as to make them useless.

throughout its frequency band.

It is a further object of the present invention to provide anauxiliarycircuitto produceacross a meter a voltage equal and opposite to thevolt- 6 Claims. (Cl. 175-183) age produced across the same meter bythenoload plate current of the oscillator so that the current through themeter'is only that current resulting from loading of the plate circuit`of the oscillator by a load coupled thereto.

Other objects will become apparent upon reading the specication andinspection Yof the figures of the drawing and will be specically pointedout in the claims.

Referring to the drawing:

Figure 1 is a circuit diagram of the invention.

Figure 2 is the current-frequency relationship of the invention. f

Referring more specifically to Figure 1, 2 is an electronic tube hereillustrated as a triode having a plate 4, a control grid 6, anindirectly heated cathode 8 and a heater I0. A triodeis' used forsimplicity of illustration only and any other suitable oscillationgeneratorv may be substituted therefor Without departing from the scopeof this invention.

The grid circuit of the tube includes bias resistor I2, couplingcondenser I4 and inductance 'I'he plate circuit of the oscillatorincludes an inductance I8 which is coupled to inductance I6 in such aphase as to feed back energy and maintain a status of oscillationthroughout" the freouency range. Variable condenser 20 is connected inparallel' with inductance I8 to produce a resonant circuit which istunable through a large frequency range. Condenser 22 is a condenser topermit the passage of alternating current and to block D. C. potentialfrom the cathode. The load to be driven, the magnitude of which it isdesired to measure, may be coupled to the terminals 24 and 26. This loadis shown in schematic form as 28 and may be either an electrical orelectro-mechanical load. In one specific use of this invention this loadis a transducer of the piezoelectric crystal type. 30 is hereschematically shown as a D. 'C.' power supply receiving energy fromlines 32 and 34. This power supply forms no part ofthe present inventionand is shown for the sake of completeness only. It is, however,necessary that this power supply be of the constant potential type sothat no variation in potential at the D. C. output terminal results fromthe normal current requirement fluctuations. 36 is a highly sensitivecurrent meter. This meter, in the normal operation of this device, hasno current iiowing therethrough under no-load conditions. 38 is anauxiliary constant voltage source of D. C. potential. I t is verynecessary that the terminal voltage of this source of potential remainconstant under different current requirements. l0 nis a nonuniformlywound resistor or any suitable resistor having a resistance which variesin the desired manner as will be pointed out later in the specification.42 is the potentiometer arm cooperating with this resistor and isrigidly coupled to the tuning plates of the condenser 20. If it isdesired to use permeability tuning of this oscillator, this arm 42 wouldbe coupled to the movable core of the permeability tuned circuit.

Referring more specifically. to Figure 2, the frequency-currentrelationships of this invention are illustrated. The frequency rangeofthe device varies from a low frequency of datoithe high frequency of50. Asthe device is tuned over this frequency band 52 is the magnitudeof the normal no-load current vand 54 isthe `magnitude of thecompensating current developed by potential source 38 and its associatedcircuit. The resulting no-load current through the meter 3671sthe,algebraic sum of thenormal no-load plater current andv the compensatingcurrent or zero.' This zerometer current `is illustrated as 56;Thezoperation of thisdeviceis asfollows-n The oscillator. generates asignal the wave length ofwhich is` determinedV by the resonant frequencyof the plate circuit. Depending on theimpedance ofl thiscircuit andother characteristics of the system a certain rie-load plate current iscaused toflow through-the conductor Ellffrornr the power supply 3d. ThisVcurrent will vary with the impedance Vof the plate circuit andtherefore vary as the oscillator is-tuned over its frequencyV range. Itisf desirable that the noloadcurrent-through the meter 36 beof minimumvalueand preferably zero Vin order that aV meter of. high sensitivityand low range be used. In order tocompensate for or counteract thecurrentlnormally` flowing throughthe rneterv 3S the auxiliaryA circuitincluding the resistor and the source of potential 33 is introduced. Thepotential introduced atV the terminals llt and Miu by this auxiliarysource of voltage is equal andfopposite-polarity to that voltageproduced at. these` terminals by the no-loadplate current. To.get-perfectncancellation it is therefore necessaryv thatt the resistort0` be of` the required configuration ton generatetheAV necessarypotential acrossterminals lll and 461as-the-contact 2. is movedinunisonwith thetuning. element of the platecircuit.. If. theelements 38,40and42.. are all..tailor.-made to the correct values the meter 36 will.havezero current flowing through itat allfrequencies. when` underno-load condition. After. the adjustmentsV are. madeY for the -no-loadconditions, theload, hereshownas 2.8, is coupled `to .the-,platecircuit.. T-he meter. is .not compensated-for plate currentresultingfrom thechange of impedance in the. platecircuit caused by the load. Forthis reason the.. meter will. now read thechange in platecurrentresulting froinload only. Fromthe abovedescription voftheapparatusand' its methodof. operation it will be.. seen that. the meter.36., measures only the. load currentand therefore it. may have a quite.small current range with vhigh resultingsensitivity.

Itis to be understood also that `although the invention-has been`described with specific reference: to a'particular embodiment' thereof;it is not'to. b'esolimited, sincechanges and alterations thereinmay bemade. which are within the fullintended scope ofr this inventionasdenedlby the appended' claims.

'If claim:

' 1; In ari-electronic oscillator theresonant frequency of which may bevaried, a direct current source of plate potential for said oscillator,a tunable plate circuit in said oscillator for varying the resonantfrequency of said oscillator, a meter connected in said plate circuitresponsive to plate current, a second source of potential connected inparallel with said meter, and automatic coupling means for varying saidsecond source simultaneously with said resonant frequency whereby saidmeter is compensated for changes in plate current resulting from varyingsaid resonant frequency.

2. An electronic oscillator for driving an electrical'loadincluding, aplate circuit tunable over a` frequency band by a variable condenser, asource of plate potential in said plate circuit, a. direct currentmeasuring device in said plate circuit, and an automatic compensatingcircuit for said measuring device, including a second source ofpotential connected in parallel with said measuring deviceV and inopposite polarity to said source of plate potential and means forvarying said second source of potential simultaneously with saidvariable condenserV whereby said measuring device is compensated forchanges inV current resulting from change in oscillator resonantfrequency as said plate circuit is tuned.

3. In an electronic oscillator having a tunable plate circuit, avariable electrical reactance element for adjusting the resonantfrequency of said plate circuit, a source of plate potential for saidoscillator, a self compensated circuit in said plate circuit including asecond source of potential and means for varying said second source ofpotential simultaneouslywith said reactance element to therebycompensate for variations in plate current normally resulting from thechange in plate circuit impedance caused by said tuning.

4. In an electronic oscillator having a tunable plate circuit, means forvarying the resonant frequency of said plate circuit, a source of platepotential for said oscillator, and a self compensated circuit in saidplate circuit, including a second source 0f potential, a4 variableresistor for varying the voltage output of said second source ofpotential, and an operative connection between said variable resistorand said means for varying the resonant frequency to thereby causevariations in said voltage output as said plate circuit is tuned wherebythe variation in plate current normally resulting from said tuningisminimized.

5. In an electronic oscillator having a tunable plate circuit, avariable electrical reactance element for adjusting the resonantfrequency of said plate circuit, a source of plate potential for saidoscillator, and a self compensated circuit in said plate circuit,including a second source of potential, a variable resistor for varyingsaid second source ofV potential, and an operative connection betweensaid variable resistor and said variable reactance adapted to causesimultaneous variations in said second source of potential as saidreactance is varied to thereby compensate for variations in platecurrent normally resulting from the change in plate circuit impedance assaid reactanceelement is varied.

6. In an electronic oscillator having a tunable plate circuit, means foradjusting the resonant frequency 0f said plate circuit, a source ofplate potential for said oscillator, and automatic compensating means insaid plate circuit, including aV variable secondary source of potential,and means coupling said variable secondary source to said frequencyadjusting means thereby causing variations in said secondary source assaid plate circuit is tuned to compensate for variations in platecurrent normally resulting from said tuning.

WESLEY S. ERWIN.

REFERENCES CITED The following references are of record in the Number 6Name Date Burford Nov. 19, 1940 Deal et a1 May 27, 1941 Allison Sept. 9,1941 S-lonczewski Nov. 11, 1941 Schock Apr. 28, 1942 Tubbs Aug. 18, 1942Elmendorf et al. Aug. 10, 1943 Loughlin Dec. 28, 1943 OTHER REFERENCESVacuum Tube Voltmeters, Rider, October 1945, 11th printing, John F.Rider Publisher, Inc., pages 55 and 56.

